Layered and homogeneous films of aluminum and aluminum/silicon with titanium and tungsten for multilevel interconnects

ABSTRACT

Layered structures (e.g., Al-Si/Ti/Al-Si . . . ) and homogeneous alloys of aluminum and aluminum/1 at. % silicon with titanium and tungsten and other refractory metals have been found to significantly reduce hillock densities in the films when small amounts of titanium or tungsten are homogeneously added. However, the resistivity of the films can become excessive. In addition, a new type of low density hillock can form. Layering of the films eliminates all hillocks and results in films of low resistivity. Such layered and homogeneous films made with Al-Si and Ti were found to be dry etchable. Electrical shorts in test structures with two levels of metal and LPCVD SiO 2  as an interlayer dielectric have been characterized and layered films using Al-Si and Ti gave excellent results.

This invention was made with Government support under contractMDA-903-84-K-0062 awarded by the DARPA. The Government has certainrights in this invention.

This invention is directed to the field of semiconductor devices, andmore particularly to films useful in interconnects in integratedcircuits.

With advances in integrated circuit technology, device dimensions arebeing scaled down and concurrently the chip size and complexity arecontinually increasing. Since the smaller size of devices makes themfaster, circuit performance should improve. However, the highercomplexity and larger chip size require closely spaced, longinterconnecting lines. As a result, the RC time delay, the IR voltagedrop, the power consumption and cross-talk noise associated with theinterconnection lines can become appreciable. Thus, even with very fastdevices, the overall performance of a large circuit can be seriouslyaffected by the limitations of the interconnections.

To accommodate the needs of future VLSI technology, new materials mustbe investigated for use in fabricating multilevel interconnections. Inthis application, the term, “level” will be used to describe conductorswhich are separated by an insulator; the term, “layer” will be used todescribe different conductors together at one level of interconnection.

For a long time, aluminum has been used to form the metal ininterconnections; however, as device dimensions are scaled down, thecurrent density increases, resulting in a decrease in reliability. Someof the problems with pure aluminum are electromigration, high solubilityand diffusivity of silicon leading to poor contract reliability toshallow junctions, and hillock formation causing electrical shortsbetween successive levels of aluminum. Such hillock formation is anespecially notable problem in multilevel interconnection films, causingshorts between levels.

However, aluminum is preferred over most other metals forinterconnection structures because of its low resistivity and siliconcompatibility. Tungsten has been used for interconnections, and it hasbeen urged that the resistance of CVD tungsten can be as good asdeposited aluminum when step coveraged is considered. However, variousplanarization processes can be used to overcome the increasedresistances associated with thickness reduction and steps.Aluminum-copper is sometimes used to solve problems characteristic ofpure aluminum. However, it is known that it is difficult to dry etchthis combination; the combination corrodes easily; and, hillocks are notcompletely eliminated. In addition, it has been found that copperrapidly diffuses through SiO₂ degrading underlying devicecharacteristics.

Studies have been done on the problem of hillock formation due toelectrical and/or thermal stress. Hillocks form in part because of largedifferences between the thermal expansion coefficients of Al and Si. Oneknown method of reducing these hillocks is to deposit a film beneath theAl which has an expansion coefficient between that of Al and Si. This isusually done with silicides such as WSi₂ or MoSi₂. It was also triedwith a Ti-W alloy as the bottom layer, but with no noticeableimprovement in hillock density, although an increase in electromigrationlifetime has been reported.

It is an objective of the present invention to provide an improved filmfor use in providing interconnections in an integrated circuit.

It is a further objective of the present invention to provide animproved film incorporating aluminum for use as an interconnection levelin multilevel interconnection structures used ion integrated circuits.

It is another objective of the present invention to provide a multilayerinterconnection film incorporating aluminum which has reduced hillocksin order to minimize breakdown in multilevel interconnection orcapacitor structures in integrated circuits.

Another objective is to provide a structure which is dry etchable, tofacilitate processing.

Yet another objective is to provide smooth conductive films havingreduced electrical resistivity.

These and other objectives of the present invention are achieved byfabricating a film for a VLSI interconnection incorporating Al-Si whichmay be alternatively layered with refractory metals at eachinterconnection level or which may incorporate a refractory metal in ahomogeneous level for a multilevel interconnection structure. It hasbeen found that titanium (Ti) is especially useful in forming suchstructures; tungsten was also tested with some limited success. It isspeculated that Zr, Hf, V₂ and Ta will also prove to be satisfactory.

It has further been found that a sputtering technique is especiallyuseful in laying down such films. The success of this technique hasbaffled many prior researchers in this field. Prior researchers havetried to alloy aluminum with metals such as titanium and have found thatthe resulting film have very high resistivity, making it unsuitable.This research has demonstrated that aluminum, silicon and titaniumtogether resulted in useful films. The key features of this inventioninclude the addition of both silicon and titanium to aluminum to providethe film, and the discovery that such a film may be laid down either asa homogeneous film or as a layered film comprising alternate layers ofaluminum-silicon and the refractory metals such as titanium. Theinvention will be better understood with respect to the followingfigures wherein:

FIGS. 1A, 1B and 1C are cross sections schematic of homogeneous andlayered structures incorporating the present invention;

FIG. 2 comprises SEM photomicrographs, surface profiles and resistivityof pure Al and Al-Cu-Si, showing hillocks;

FIG. 3 comprises SEM photomicrographs, surface profiles and resistivityof homogeneous and layered films using Al-Si and Ti;

FIG. 4 is a chart of failures by shorting in a two-level metalinterconnection test structure as a function of atomic percentage of Ti;

FIG. 5 comprises a chart showing resistivity of homogeneous Al-Si-Tifilms as a function of atomic percentage Ti;

FIG. 6 shows mechanical stress versus measurement temperature of Al-Siand Al-Si-Cu films;

FIG. 7A shows stresses in homogeneous Al-Si-Ti films versus measurementtemperature;

FIG. 7B shows stresses in layered Al-Si with Ti films versustemperature;

FIGS. 8A and 8B comprises surface profiles of various Al-Si metalsystems used in conjunction with titanium;

FIG. 9 comprises surface profiles of Al-W metal systems;

FIG. 10 illustrates test results of two level metal capacitor arraysformed of at least one level (lower) of the present invention; and

FIG. 11 is a scanning electron micrograph showing micron and sub-microndry etched interconnects using Al-Si-Ti and Al-Si layered with Ti. Theequipment used in forming all of the films described in the presentapplication was a magnetron sputtering system. The films were preparedby either depositing simultaneously or layering aluminum or Al-Si withother elements. Homogeneous films can also be made from a single target.In the sputtering process, the wafers sit vertically on a drum whichrotates, passing in front of each target. The base pressure was below1.5×10⁻⁷ millibar (MBAR) and the argon gas pressure during sputteringwas 2.0×10⁻³ MBAR. To test for hillocks and resistivity, the films weredeposited on thermally oxidized silicon substrates and exposed to 450°C. annealing in H₂/N₂ gas for 30 minutes. The presence of hillocks onthe film was primarily determined by using an Alphastep surfaceprofiler. Optical and scanning electron microscopy were also used.

The resistivity of films was determined by first using a four pointprobe to measure the sheet resistance; then steps were etched, and asurface profiler was used to determine the thickness. These measurementswere done for deposited as well as annealed films.

In the preferred form of this invention, each level of a multilevelinterconnection comprises either a plurality of alternating layers ofAl-Si alternating with a refractory metal which is preferably Ti asshown in FIG. 1B; a single level of Al-Si with a level of a refractorymetal, preferably Ti, on top as shown in FIG. 1A; or a singlehomogeneous film comprising Al-Si-Ti as shown in FIG. 1C. The approachesof FIGS. 1B and 1C are the preferred embodiment. In all embodiments, thepresence of Si was essential to effective functioning of the film.

It has been found that the atomic percentage of Ti in the homogeneousfilm of FIG. 1C should be between 1 and 4 at. % with the preferred rangebeing 2-3.5 at. % and the preferred level 2.5 at. %. The histograms ofFIG. 10 which comprise short-circuit testing of capacitors fabricatedusing two-layer metalization show the improved results over the priorart using either the layered approach of FIG. 1B or the homogeneous filmof FIG. 1C. Specifically, pure Al performed quite poorly, and Al-Si-Cuhad an undesirable number of breakdowns below 120 volts. The use of 2.4at. % Ti in homogeneous layers (slightly below the preferred range)reduced breakdowns further; the use of layered Al-Si-Ti was the idealcase.

It is especially interesting to note that a classic problem with addingimpurities homogeneously to Al is that the resistivity increases. It isalso well known that addition of silicon or copper increases the filmresistivity as well as the further problems with the addition of copperof poor dry etchability and the likelihood of corrosion. Addition of Tior W homogeneously also increases the resistivity. However, if the filmsare deposited in layers as shown in FIG. 1B, the resistivity can be keptlow even after long anneals. In addition, no hillocks were observed. Noprior work has resulted in such a dramatic reduction in the presence ofhillocks. FIG. 8B shows the smooth surface profiles which result fromthe layered approach. It can be seen that Al-Si with a thin layer of Tion the bottom is somewhat more effective (FIG. 8Bii) than Al or Al-Cu,but less effective than the other approaches of the present invention.This method has been tried earlier by other researchers for purposes ofproviding barrier metals. (See R. W. Bower, Applied Physics Letters,Vol. 23, No. 2, July 1973, p. 99.) The method of FIG. 8Bii is not asdesirable as those disclosed above as shown graphically by thecomparative surface profiles of FIG. 8. A layer of Ti on top of Al-Si(FIG. 8Bi) is quite smooth and effective. Multiple layers of Tialternating with Al-Si is close to the ideal case, as confirmed by FIG.10.

To further reduce the resistivity of the layered films, thinner layersof Ti were deposited. It was found that films of Al-1 at. % Si with 100angstroms of Ti resulted in an approximately 15% reduction inresistivity over that of films using 200 angstroms. The surfacesmoothness was the same (smooth at the 20 angstrom level).

However, when Ti layers of 50 angstroms were used, the films did show alow density of hillocks. Another possible way to reduce the resistivityof the films even further would be to lower the silicon concentration inthe Al, but this may have the reverse effect because layered films ofpure Al and Ti become virtually homogeneous after 30 minutes ofannealing at 450° C. One interesting point to note is that even thoughthe films of alternately sputtered pure Al and Ti become virtuallyhomogeneous after the anneal, they are still smoother than filmsdeposited homogeneously. Therefore, one could chose a certain thicknessof Ti such as 100 angstroms and after annealing, the resulting filmwould consist of 3 at. % Ti if 100 angstroms of Ti were used for every3000 angstroms of Al.

It has also been found that if Al-Si and Ti are deposited so as to forma homogeneous film as shown in FIG. 1C, similar but slightly lessfavorable results than the layered approach described above can beobtained.

Homogeneous films of Al-Si-Ti and Al-Ti were found to exhibit differentannealing characteristics and had different resulting properties. Firstof all, it was found that if Si was not present in the Al, the surfaceswere much rougher (see FIG. 8A). A crucial difference betweenhomogeneous films with and without Si was that the resistivities of theAl-Si-Ti films were constantly lower after annealing; structures withoutSi can have resistivity values 50% higher than those with Si. Theeffects of the silicon appear to be interpretable if one assumes thiscomponent is controlling the precipitate morphology forming very smallternary precipitates and reducing the concentration of Si in the Allayers. The Ti-Al-Si ternary phase diagram shows that the solidsolubility limit of Si in TiAl₃ is much higher than that of pure Al. Italso shows that if enough Si is present, a three phase region isentered. In our experiments, films of homogeneous Al-Ti were rough, thefilms of Al-Si-Ti were smooth when the concentration was below 4.0 at. %but above this value the films were rough. Large spike like hillockswhich appear in FIG. 8Aiii begin at about 3.5 at. %.

It can be seen from FIG. 2 that hillocks constitute a major defectproblem in pure aluminum or in Al-Cu-Si film. The surface profiles whichare drawn beneath the pictures of FIG. 2 graphically illustrate thepresence of dramatic hillocks in these prior art films, and thehistograms of FIG. 10 indicate the likelihood of breakdown of capacitorsformed using such films.

According to the present invention, by using Al-1 at. % Si with up to4.0 at. % Ti, or alternatively by layering Al-Si with Ti, the hillockproblem can be virtually eliminated as demonstrated graphically in FIG.3. It was found that the number of failures due to hillocks wassignificantly reduced in the testing of two level metal capacitors asshown in FIG. 4, made with at least one level of a homogeneous Al-Si-Tifilm where the Ti was about 2.5 at. %.

The resistivity of film shown in FIG. 4 is plotted in FIG. 5, whichshows that hillock-free films can be fabricated with a resistivity of5.4 micron ohm-cm. the improvements in stress as a function oftemperature for a homogeneous Al-Si-Ti film as compared to the prior artAl-Si or Al-Si-Cu films is dramatically shown in FIGS. 6 and 7. Thedramatic improvement in surface profiles of layered and homogeneousfilms incorporating a refractory metal, preferably Ti, in Al-Si, isgraphically demonstrated in the surface profile of FIG. 8. It isbelieved that films may be effective with as little as 1 at. % or asmuch as 4 at. % Ti. Some improvement is achieved with a differentrefractory metal such as tungsten (W) as shown in FIG. 9; it is alsoapparent from these surface profiles that problems remain possibly dueto stresses.

In summary, the present invention discloses that conductiveinterconnections comprising a homogeneous film of Al-Si-Ti, oralternatively, Al-Si alternated with Ti provides hillock-free, dryetchable low resistivity electromigration resistant films. The films arealso believed to be electromigration resistant. It appears that the useof tungsten in the homogeneous or layered films may also yield improvedresults if very low concentrations are used, although titanium would bepreferable. It also appears that other refractory metals such aszirconium, tantalum, halnium, vanadium and chromium could produce goodresults. The presence of silicon in the aluminum film is necessary toachieve the desired results, because Si functions to keep the titaniumlayer intact. Finally, the resistivity of the layered films is lowerthan the homogeneous films, approaching the value for Al-Si alone.

What is claimed is:
 1. In a semiconductor device, a conductiveinterconnect level formed on a silicon substrate, including an aluminumsilicon titanium alloy wherein said titanium is 2-3.5 at. % of saidalloy.
 2. In a semiconductor device, a conductive formed on a siliconsubstrate including a homogeneous film of Al-Si with a refractory metalselected from the group consisting of titanium, tantalum, zirconium,hafnium, vanadium chromium, said titanium being 1-4 at. % of said film.3. A semiconductor device as claimed in claim 2 wherein said refractorymetal is titanium.
 4. A semiconductor device as claimed in claim 3wherein said titanium is 2-3.5 at. % of said film.
 5. A semiconductordevice as claimed in claim 3 wherein said titanium is about 2.5 at. % ofsaid film.
 6. A semiconductor device as claimed in claim 5 wherein saidalloy includes about 1% silicon by weight.
 7. An integrated circuitcomprising a silicon substrate and a contact interconnect levelcomprising a homogeneous film of Al-Si with titanium wherein saidtitanium is 1-4 at. % of said film.
 8. In a semiconductor device, aconductive level formed on a silicon substrate, including alternatinglayers of Al-Si with layers of a refractory metal.
 9. A semiconductordevice as claimed in claim 10 8, wherein said refractory metal istitanium.
 10. A semiconductor device as claimed in claim 8 wherein saidrefractory metal is chosen from the group consisting of titanium,tantalum, zirconium, hafnium, Vanadium and chromium.
 11. A semiconductordevice as claimed in claim 9 wherein the conductive level is laid downon silicon or silicon dioxide, the Al-Si layer adjoining the silicon orSiO₂ layer.
 12. A semiconductor device as claimed in claim 8 or 10wherein said conductive level is formed of repeating layers of saidAl-Si and said refractory metal.
 13. A semiconductor device as claimedin claim 9 or 11 wherein said conductive level is formed of repeatinglayers of said Al-Si and said titanium.
 14. A semiconductor device asclaimed in claim 11 including at least two layers of titanium.
 15. Asemiconductor device as claimed in claim 11 or 14 including a layer oftitanium on top of every layer of Al-Si.
 16. A device as claimed inclaim 8, wherein the alternating layers include one layer of Al-Sibetween first and second layers of the refractory metal.
 17. A device asclaimed in claim 16, wherein said refractory metal is chosen from thegroup consisting of titanium, tantalum, zirconium, hafnium, Vanadium,and chromium.
 18. A device as claimed in claim 17, wherein the one layerof Al-Si is approximately 1 at. % Si.
 19. A device as claimed in claim16, wherein the one layer of Al-Si is approximately 1 at. % Si.
 20. Adevice as claimed in claim 8, wherein at least one layer of Al-Si isdisposed on a layer of SiO ₂.
 21. A device as claimed in claim 20,wherein the at least one layer of Al-Si is approximately 1 at. % Si. 22.A device as claimed in claim 8, wherein the one layer of Al-Si isapproximately 1 at. % Si.
 23. A device as claimed in claim 8, wherein afirst pair of alternating layers includes a first layer of Al-Si and afirst layer of the refractory metal, and a second pair of alternatinglayers includes a second layer of Al-Si and a second layer of therefractory metal.
 24. A device as claimed in claim 23, furthercomprising an insulator disposed below the first pair of layers.
 25. Adevice as claimed in claim 24, wherein the one layer of Al-Si isapproximately 1 at. % Si.
 26. A device as claimed in claim 23, furthercomprising an insulator disposed between the first pair and the secondpair of layers.